The present invention relates generally to a pixel structure of a light-emitting diode (LED) display device, and more particularly to a pixel structure of an organic light-emitting diode (OLED) display device.
Current electronic display products include cathode ray tubes (CRT), plasma display panels, light-emitting diodes, and thin-film electroluminescent devices. Any display device to be accepted in the electronic display market must provide features of high resolution, full-color display with good light intensity and competitive price.
For a conventional passive organic luminescent device, although its manufacturing process is simple and manufacturing cost is less expensive, its resolution is not high. It is acceptable only for a small-area and low-resolution display device. On the contrary, an active drive organic light-emitting diode display device using thin-film-transistors (TFT) in an active-addressing scheme has features of high resolution and low power consumption. Technology using organic thin-film material to manufacture lighting device is proved to be a great advance in flat-panel display technologies. This technology has made high brightness, wide range of color, long lifetime, low temperature fabrication and low-voltage drive displays possible.
In a two-terminal scheme, the organic display medium is sandwiched between two sets of orthogonal electrodes (rows and columns). Thus, the organic electroluminescent (OEL) element serves both the display and switching functions. The current required by the OEL elements is supplied via the row and column buses. Because of the instantaneous high current, the IR potential drops along these buses are insignificant compared with the OEL driving voltage. Since the brightness-voltage characteristic of the OEL is nonlinear, any variation in the potential along the buses will result in a non-uniform pixel illumination.
U.S. Pat. No. 5,550,066 discloses a process for making an active matrix four-terminal TFT electroluminescent device in which an organic material is used as the electroluminescent medium. In this TFT-OEL display device, the IR potential drops along the buses are insignificant. Also, the uniformity of the pixel illumination is not significantly affected by the size of the display device.
FIG. 1 is a schematic diagram of a pixel structure of the conventional active matrix four-terminal TFT-OEL device. As shown in FIG. 1, each pixel structure 100 comprises two thin film transistors 101 and 102, a storage capacitor 103 and an OEL pad 104 arranged on a substrate. The drain electrode of the TFT 102 is electrically connected to the OEL pad 104. The source electrode is electrically connected to the ground bus 107. The gate electrode is electrically connected to the storage capacitor 103. The TFT 102 is the logic transistor with the source bus as the data line 105 and the gate bus as the gate line 106. The drain electrode of the TFT 101 is electrically connected to the gate electrode of the TFT 102 which in turn is electrically connected to the storage capacitor 103 so that following an excitation signal the TFT 102 is able to supply a nearly constant current to the OEL pad 104 between signals.
However, because the OEL element is driven by current, if a TFT is used to supply current to the OEL element and to serve as an active drive element in order to form a type of an active matrix drive, then the uniformity of the TFT-driven current on the glass substrate of the display becomes very important. For low-temperature poly Si (LTPS) thin film transistors, the pixel illumination of the OEL element is very non-uniform under the condition of poor control of the driving current.
The present invention has been made to overcome the above-mentioned drawbacks of a conventional organic light-emitting display device. The primary object of the invention is to provide a pixel structure of an OLED display device. The pixel structure of an OLED display device connects a resistor in series with the source electrode of a thin film transistor to increase the illumination uniformity of each pixel of the OLED display device.
The pixel structure of an OLED display device of the invention comprises mainly first and second thin film transistors, a storage capacitor, a resistor and an OEL element. The resistor is connected in series with the source electrode of the second thin film transistor. It can be active or passive.
According to the invention, a negative feedback loop is formed by the connection of the resistor in series with the source electrode of the second TFT. The illumination uniformity of every pixel of the OLED display device is then improved.
In the preferred embodiments of the invention, the second TFT can be a p-type or an n-type thin film transistor. In this invention, the relationship between the current of the drain electrode of the second TFT and the voltage of the gate electrode of the second TFT is analyzed by connecting a resistor of various resistance levels in series with the source electrode of the second TFT. When the current of the OLED is within the micro-amp level and the resistance level of the resistor is within the million-ohm level, the current-voltage characteristic curves have more linear relationship and the switching of gray-level brightness becomes easier.
The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings.